Shaker tables are used to test objects under conditions of violent shaking, such as parts of a missile that will be subjected to large vibrations in use, or scale models of buildings or other structures that may be subjected to violent shaking in an earthquake. A shake table apparatus may include a table on which a test object is mounted, actuators for shaking the table, and couplings for connecting the actuator to the table. In many tests, the table is shaken not only in a vertical or Z direction, but in perpendicular horizontal X and Y directions. Accordingly, the couplings that connect the vertical actuators to the table, must permit some horizontal table movement while the table is shaken vertically.
For actuators of given capacity, the mass that can be shaken to a given acceleration level, or the acceleration level that can be achieved for a given mass to be tested, depends to a large extent upon the mass of the couplings and the bearing assemblies thereof. A common type of bearing assembly includes upper and lower bearings that each have inner and outer engaged spherical members. A shaft connects the upper and lower inner members, and an outside transmit member couples the upper and lower outer members. The bearing assemblies constitute a large part of the mass that is shaken. In one example, the test object weighs 150 pounds, the table weighs 400 pounds, and each of four bearings that couple four vertical actuators to the table, weighs 300 pounds. Thus, the mass of 1200 pounds for the four bearing assemblies constitutes a major portion of the total mass to be shaken vertically. Four additional bearing assemblies that couple four horizontal actuators to the table, have portions that move vertically with the table and therefore constitute additional mass that must be shaken vertically. If the mass of the bearings could be reduced, this would allow a shake table apparatus with the same actuators and table, to be shaken at much greater acceleration levels, or allow a much larger object to be shaken at a given acceleration level.